Yea, if you're talking full-frame it's worth a strong consideration. I'd get it in a heartbeat if I didn't already have 14mm covered. On crop sensors the Tokinas and Nikons are better (build and image quality) and cheaper and faster.

I'll admit being biased though. The Tokina AT-X Pro series of lenses is the only third-party lenses I've ever been so impressed with. I shot quite a bit with most the crop ultrawides before buying my 11-16, and while the Sigma 12-24 feels much better than the Sigma 10-20 (and seems to flare less) (and totally smokes the crapfest Tamron 11-18 (never tried the Tamron 10-24)) it still felt inferior.

freedom7 said:
Anyone know what the quality is like on the super wide Sigma 12-24?

It's a great lens for the money. It covers full frame!

Wide open, it is sharp in the centre and soft at the edges. It performs best stopped down to f/8-f/11, when it is sharp across the frame. Contrast is good and it is surprisingly resistant to flare for such a wide angle of view.

Its particular strength is its lack of rectilinear distortion. Effectively, there is none. Sigma has made a remarkable job of correcting distortion that could reasonably be expected to be severe.

The weakness, as always, is Sigma's variable build quality. If you get a good one, it's a fine lens. If you get a bad one, it will probably fail just when you need it not to.